Logic gates

                               -:Logic gates:-

Basic gates:-

There are three basic gates in digital electronics. These are NOT gate, AND gate & OR gate.

NOT gate / Inverter :-

NOT gate is called inverter also. Operation of NOT gate is if input is high output is low and if input is low then output is high. Another words output is always inverse or toggle of input. So it is called inverter. Circuit diagram of inverter is given below

Here input is A output :-

Truth table of NOT gate:-

From truth table we can say that NOT gate shows output high if the input low and shows output low if the input high. Example:    A is an input signal for NOT gate in black color and X is the output signal in red color. It is clear NOT gate always reverses its input signal.

Truth table of NOT gate:-

OR Gate:-

The output is high if any input is high for OR gate operation. Only output is low when all input is low. If input A, B and output is X. Then the operation is written in with plus (+).

Mathematically X = A+B.

Symbol of OR Gate:-

Truth table of OR gate :-

Here two inputs are shown in the table. For first case where two inputs are low, output also low. But all outputs are high because any one input is high. Example: A and B are the input here. If one input is high then output is high. Output wave X shows that.

Truth table of OR gate:-

AND gate:-

When all inputs are high output is high in AND gate. If one input is low then output low. Diagram of AND gate is

Symbol of AND Gate:-

Truth table of AND gate:-

AND operation is written multiple with the inputs. For input A, B and output X the operation is X=AB or X=A.B Example:  From output signal where the two inputs are high output also high.  Otherwise all area are low in AND gate.

Truth table of AND gate:-

NOR Gate:-

NOR gate is opposite of OR gate. NOR gate gives high output when all inputs are low. Diagram of NOR gate approximately same only one change that a single bubble used in output bar.

Symbol of NOR Gate:-

Truth table shows only when all inputs are low then output high. Example:  NOR gate produces low when any one of input is high.

Truth table of NOR gate:-

NAND Gate:-

NAND gate also opposite of AND gate. NAND gate produces high if all input are low otherwise all output are high. Diagram of NAND gate similar to AND gate only one change that is one bubble used to output terminal.

NAND operation written in:-

Truth table:-

                 

XOR Gate:-

XOR gate means exclusive OR gate. When both inputs are opposite logic level only then output are high. 

Expression of XOR gate written as  alternatively written:-

Truth table:-

Logic Families:-

•Logic Family : A collection of different IC’s that 

have similar circuit characteristics.

•The circuit design of the basic gate of each logic 

family is the same.

•The most important parameters for evaluating 

and comparing logic families include :

–Logic Levels.  

–Power Dissipation.                                   

–Propagation delay.                                                                                

–Noise margin.

–Fan-out ( loading ).                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
                                                             


               
               Example Logic Families:-

                    

•General comparison or three commonly 

   available logic families.

Implementing Logic Circuits:-

• There are several varieties of transistors – 

the building blocks of logic gates – the most 

important are:

–BJT (bipolar junction transistors)

•one of the first to be invented

–FET (field effect transistors)

•especially Metal-Oxide 

Semiconductor types (MOSFET’s).

•MOSFET’s are of two types:  NMOS 

and PMOS.

TTL and CMOS:-

•Connecting BJT’s together gives rise to a 

family of logic gates known as TTL.

•Connecting NMOS and PMOS transistors 

together gives rise to the CMOS family of 

logic gates.

Electrical Characteristics:-

•T T L

–faster (some versions).

–strong drive capability.

–rugged.

•C M O S

–lower power consumption.

–simpler to make.

–greater packing density.

–better noise immunity.

• Complex IC’s contain many 

millions of transistors.

• If constructed entirely from TTL 

type gates would melt.

• A combination of technologies 

(families) may be used.

• CMOS has become most popular 

and has had greatest development.

Logic Level & Voltage Range:-

•Typical acceptable voltage ranges for 

positive logic 1 and logic 0 are shown 

below.

•A logic gate with an input at a 

voltage level within the 

"indeterminate" range will produce 

an unpredictable output level.

Characteristics: TTL and MOS:-

Remember:-

•TTL stands for Transistor-Transistor Logic.

–uses BJTs.

•MOS stands for Metal Oxide 

 Semiconductor.

–uses FETs.

•MOS can be calassified into three sub-

families :-

– PMOS (P-channel).

– NMOS (N-channel).

– CMOS (Complementary MOS, most common).

Transistor-Transistor Logic Families:-

•Transistor-Transistor Logic Families:

–74L    Low power 

–74H    High speed 

–74S    Schottky 

–74LS    Low power Schottky 

–74AS    Advanced Schottky 

–74ALS    Advance Low power Schottky

CMOS Logic Families:-

•CMOS Logic Families

–40xx/45xx  Metal-gate CMOS.

–74C  TTL-compatible CMOS.

–74HC  High speed CMOS. 

–74ACT  Advanced CMOS -TTL 

  compatible.

                                 " Thank You "